Conventionally, it has been common that this kind of the document fastener is made of metal, however, it is difficult to sort and discard the document fastener and it requires care to deal with the document fastener because its edge is sharp. Then a binder made of resin is recently conceived. For example, the binder disclosed in the patent document 1 comprises a body part extending longitudinally and a pair of folded pieces extending from the body part, and can hold leaves in a bound state by engaging a part of the folded piece with a part of the body part by making use of a concave/convex structure with forming a loop shape as a whole.
However, with the arrangement of the above-mentioned patent document 1, a mold to form a concave/convex structure is costly and it is not suitable to repeat a process of adding or removing leaves with frequency because an operation of engaging or disengaging the folded piece with or from the body part requires some degree of a knack and a force of a fingertip as well.
Meanwhile, as shown in the patent document 2, a document fastener using a strip-shaped body made of drawn polyolefin resin, more preferably a strip-shaped body made by rolling and drawing the drawn polyolefin, has been devised. Since it is possible for this kind of the document fastener to keep a folded state to a certain degree in spite of the resin because an angle to return from the folded state is small, the document fastener can effectively acts as a binder. In addition, the document fastener is easy to handle and it becomes easy to sort and discard the document fastener.    Patent document 1: Japan patent laid open number 11-78336    Patent document 2: Japan patent laid open number 2005-67027